Vibrational analyses of 1,3-dibenzoyl-4,5-dihydro-1H-imidazole-2-thione and 1,3-dibenzoyl tetrahydropyrimidine-2(1H)-thione were carried out using normal coordinate analysis. FT-IR spectra were recorded in the region 4000-400 cm(-1). The harmonic vibrational frequencies, molecular geometry and atomic charges have been computed, and NBO analysis has been carried out with the help of B3LYP density functional theory (DFT). The computed geometrical bond lengths and bond angles agree well with the crystallographic data. Atomic charges based on Mulliken population analysis, natural population analysis. Hirshfeld-I analysis and CHelpG analysis were calculated using the basis sets of 6-31G* and 6-31G**. Stabilities of the two molecules were analyzed by means of natural bond orbital (NBO) analysis and delocalized pi-pi* interactions. (C) 2012 Elsevier B.V. All rights reserved.